Anodization treatment method for aluminum alloys containing cooper

ABSTRACT

A method of treating a part made of an aluminum alloy containing copper at a content in the range 0.1% to 10% by weight includes providing the part; and performing electrochemical pretreatment of the part in a first electrolyte bath containing sulfuric acid and a first oxidizer compound, a first potential difference being established between a first cathode and a first anode dipped in the first bath, the part being the first anode, the concentration of the first oxidizer compound being such that the corrosion potential of the aluminum alloy is greater than +100 mV relative to a hydrogen normal electrode. After the pretreatment, the method includes anodizing the part in a second electrolyte bath containing sulfuric acid and a second oxidizer compound, a second potential difference □V2 being established between a second cathode and a second anode dipped in the second bath, the part being the second anode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from French Patent Application No. 1256393, filed Jul. 4, 2012, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method of treating an aluminum alloy part containing copper at a content lying in the range 0.1% to 10% by weight. Aluminum alloys containing copper corrode essentially in localized corrosion manner in the form of corrosion points or pits. This phenomenon occurs at the locations of copper-rich particles that are present in such alloys.

In order to improve the corrosion resistance of aluminum alloys in general, anodization (or chemical conversion) is performed on such alloys to increase the thickness of the layer of oxides (aluminum oxide with anodization) on their surfaces. Before receiving such anodization treatment (or chemical conversion treatment), such alloys need to be pretreated. Such pretreatment has the following objectives:

-   -   removing dirt from the surface together with residual oxides         without dissolving the underlying metal (degreasing/descaling         method);     -   smoothing the surface of the metal by dissolving a portion of it         (burnishing method).

The pretreatment is usually performed in an aqueous electrolyte by a chemical technique or an electrochemical technique. The electrolyte contains one or more acids such as sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄). Adding one or more oxidizers such as nitric acid, hydrogen persulfate, hydrogen perborate, and hydrogen peroxide H₂O₂ to the electrolyte encourages dissolution of the metal and emulsification of the surface dirt or grease, and makes it possible for the aluminum alloy parts to be rinsed acceptably. With burnishing methods, adding an organic compound leads to a viscous layer being formed on the surface of the aluminum alloy, thereby contributing to dissolving surface irregularities.

During the pretreatment of aluminum alloys containing copper, it is observed firstly that the copper-rich particles dissolve in part only, thereby leading to the presence of numerous defects (holes, cracks, cavities) in the oxide layers that are subsequently formed on the surface of the alloy by anodizing (or by chemical conversion), these defects weakening such layers and leading to premature corrosion of the alloy.

It is also observed that the copper present in the bath becomes redeposited very quickly in the form of metallic copper on the surface of the alloy, thereby likewise leading to premature corrosion of the alloy.

The present invention seeks to remedy those drawbacks.

SUMMARY

An embodiment of the invention seeks to propose a method of treating a copper-containing aluminum alloy with pretreatment and anodizing in which copper particles are dissolved completely, and in which it is possible to avoid copper subsequently being redeposited on the surface of the aluminum alloy, the method also being suitable for minimizing dimensional variations of the part.

This aspect is achieved by the fact that the method comprises:

a. providing said part;

b. performing electrochemical pretreatment of the part in a first electrolyte bath containing sulfuric acid and a first oxidizer compound, a first potential difference ΔV1 being established between a first cathode and a first anode dipped in the first bath, the part being the first anode, the concentration of the first oxidizer compound being such that the corrosion potential of the aluminum alloy is greater than +100 millivolts (mV) relative to a hydrogen normal electrode;

c. after step b, anodizing the part in a second electrolyte bath containing sulfuric acid and a second oxidizer compound, a second potential difference ΔV2 being established between a second cathode and a second anode dipped in the second bath, the part being the second anode.

By means of these provisions, the surface of the part is cleaned, copper-rich particles are removed from the surface, and subsequent redeposition of copper particles is prevented, thus making it possible to obtain an oxide deposit by anodization that is of better quality. The part is thus better at resisting corrosion.

Furthermore, the cleaning of the surface of the part removes sufficiently little material to ensure that the dimensional variations of the part remain within acceptable tolerances.

Beneficially, the first potential difference ΔV1 lies in the range 3 volts (V) to 12 V.

Thus, the pretreatment of the part is faster and leads to better quality pretreatment and then to better subsequent anodizing of the part.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood and its benefits appear more clearly on reading the following detailed description of an implementation given by way of non-limiting example. The description refers to the accompanying drawing, in which:

FIG. 1 is a diagram showing the steps of the method of an embodiment of the invention; and

FIG. 2 is a diagram showing the steps of another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A part 50 is provided that is made of an aluminum alloy containing copper (step a of the method of an embodiment of the invention). The content of copper in the alloy lies in the range 0.01% to 10% by weight.

A first electrolyte bath 10 is provided that contains sulfuric acid and a first oxidizer compound. This first bath 10 is contained in a first vessel 15. A first cathode 11 and the part 50 are dipped in this first bath 10, with the part 50 acting as a first anode.

By way of example, the first cathode 11 is made of titanium alloy or of lead alloy or of stainless steel.

The part 50 is subjected to electrochemical treatment in the first bath 10 by applying a first potential difference ΔV1 between the first cathode 11 and the part 50. This treatment constitutes the electrochemical pretreatment of the method of an embodiment of the invention (step b).

The concentration of the first oxidizer compound is selected in such a manner that during this electrochemical pretreatment the corrosion potential of the aluminum alloy of the part 50 is greater than +100 mV relative to a hydrogen normal electrode.

Tests performed by the inventors show that under such circumstances copper-rich particles are dissolved completely while the quantity of alloy that is dissolved is sufficiently small for the dimensional specifications of the part 50 to continue to be satisfied. Thus, the quantity of alloy that is dissolved is less than 0.1 milligrams per square decimeter per minute (mg/dm²/min).

Furthermore, during the electrochemical pretreatment of an embodiment the invention, no copper is redeposited on the part 50.

In contrast, when performing prior art chemical pretreatment (without electrolysis), the quantity of dissolved alloy lies in the range 0.4 mg/dm²/min to 4 mg/dm²/min.

The concentrations of sulfuric acid and of the first oxidizer compound needed to ensure that the corrosion potential of the aluminum alloy of the part 50 is greater than +100 mV depend on the aluminum alloy and on the nature of the first oxidizer compound.

Tests performed by the inventors show that for a sulfuric acid concentration lying in the range 70 grams per liter (g/L) to 250 g/L, the concentration of the first oxidizer compound needs to be greater than 0.1 moles per liter (mol/L).

For example, if the first oxidizer compound is NaBO₃, its concentration should lie in the range 0.1 mol/L to 0.5 mol/L.

For example, if the first oxidizer compound is K₂S₂O₈, its concentration should lie in the range 0.1 mol/L to 0.5 mol/L.

For example, if the first oxidizer compound is hydrogen peroxide H₂O₂, its concentration should lie in the range 0.1 mol/L to 1 mol/L.

Beneficially, the first oxidizer compound is hydrogen peroxide.

Tests performed by the inventors show that hydrogen peroxide makes it possible to reach a corrosion potential of the aluminum alloy of the part 50 relative to a hydrogen normal electrode that is greater than the corrosion potential obtained with some other oxidizer compound.

Beneficially, the first potential difference ΔV1 lies in the range 3 V to 12 V.

When the first potential difference ΔV1 is less than 3 V, the dissolution of the aluminum alloy of the part 50 is too slow for the production rates desired for the parts.

When this first potential difference ΔV1 is greater than 12 V, then an anodized layer is formed on the surface of the part 50 too quickly to enable the particles of copper to dissolve and move away from the surface of the part 50.

Beneficially, the first potential difference ΔV1 lies in the range 5 V to 10 V.

In an embodiment of the invention, the duration of the pretreatment lies in the range 2 minutes (min) to 30 min.

After performing the above-described electrochemical treatment of the part 50, the part 50 is anodized in a second electrolyte bath 20 containing sulfuric acid and a second oxidizer compound. This second bath 20 is contained in a second vessel 25. A second cathode 21 and the part 50 are dipped in the second bath 20 with the part 50 acting as a second anode.

By way of example, the second cathode 21 is made of a lead alloy or of a titanium alloy or of a stainless steel.

The part 50 is subjected to electrochemical treatment in the second bath 20 by applying a second potential difference ΔV2 between the second cathode 21 and the part 50. This treatment constitutes the anodizing of the method of an embodiment of the invention (step c).

By way of example, the second oxidizer compound is selected from a group constituted by NaBO₃, K₂S₂O₈, and hydrogen peroxide H₂O₂.

At the end of the method of an embodiment of the invention, an aluminum oxide layer is obtained on the part 50 by anodizing, which layer presents corrosion resistance that is greater than that obtained by a prior art method. Thus, tests performed by the inventors on aluminum alloys 2214 and 7050 (after sealing the oxide layer and after 500 hours exposure to a neutral saline mist) show that the number of pits per square decimeter (dm²) is less than 1 when using the method of the invention, whereas the number of pits per dm² is greater than 5 (for the 2214 alloy) or greater than 10 (for the 7050 alloy) when using a prior art method with chemical pretreatment.

The nominal chemical composition of the 2214 alloy (standard EN 573-1) is as follows: Si: 0.5-1.2; Fe: 0.3: Cu: 3.9-5; Mn: 0.4-1.2: Mg: 0.2-0.8; Cr: 0.1; Zn: 0.25; Ti: 0.15; impurities: 0.15; the balance being Al.

The nominal chemical composition of the 7050 alloy (standard EN 573-1) is as follows: Si: 0.12; Fe: 0.15: Cu: 2-2.6; Mn: 0.1; Mg: 1.9-2.6; Cr: 0.04; Zn: 5.7-6.7; Zr: 0.08-0.15; Ti: 0.6; impurities: 0.15; the balance being Al.

Tests performed by the inventors show that the anodization layer that forms on the surface of the part 50 during anodizing provides even better protection (the impedance of the anodization layer is higher) when the prior electrochemical pretreatment (step b) is performed with a first potential difference ΔV1 of 10 V for 5 min or 5 V for 10 min.

Tests performed by the inventors show that the anodization layer that forms on the surface of the part 50 during anodizing forms better protection (its impedance is higher) when the hydrogen peroxide concentration lies in the range 5 g/L to 25 g/L for a 7050 aluminum alloy.

Tests performed by the inventors show that the anodization layer that forms on the surface of the part 50 during anodizing offers better protection (its impedance is higher) when the hydrogen peroxide concentration is greater than 2 g/L for a 2214 aluminum alloy.

Anodizing is performed with a sulfuric acid concentration lying in the range 100 g/L to 300 g/L depending on the looked-for thickness of the anodization layer.

Beneficially, the concentration lies in the range 160 g/L to 240 g/L, leading to better efficiency for the method.

Still more beneficially, this concentration is approximately equal to 200 g/L.

Beneficially, the second oxidizer compound is hydrogen peroxide.

Beneficially, the anodizing is performed with a hydrogen peroxide concentration lying in the range 15 g/L to 20 g/L.

Beneficially, anodizing is performed with a voltage pause, i.e. a first voltage is applied lying in the range 12 V to 17 V, and then optionally a second voltage is applied lying in the range 17 V to 22 V in order to avoid damaging the electrical installations.

In a first implementation of the invention, the part 50 is rinsed between pretreatment step b and anodizing step c.

Pretreatment thus takes place in a first bath 10 that is distinct from the second bath 20, the first vessels 15 being distinct from the second vessel 25.

After pretreatment, the part 50 is extracted from the first bath 10 and rinsed. This rinsing presents the benefit of removing any residues that might be present on the surface of the part 50 at the end of the pretreatment.

In addition, since the second bath 20 is distinct from the first bath 10, the second bath is not polluted by elements that were dissolved during the pretreatment and that are present in the first bath 10.

Once the part 50 has been rinsed, the part 50 is dipped in the second bath 20 so as to be subjected to the anodizing treatment therein.

Beneficially, the part 50 is rinsed by performing the following steps:

b1. rinsing the part 50 in a static rinsing bath 91; and

b2. then rinsing the part 50 in a running rinsing bath 92 with softened water.

This makes the rinsing of the part 50 more effective.

In another implementation of the invention, anodizing step c takes place in the same bath as the first bath 10 of pretreatment step b, without extracting the part 50 from the first bath 10 between step b and step c.

The second vessel 25 is thus the same as the first vessel 15, and the second bath 20 is constituted by the first bath 10 as it exists at the end of the pretreatment.

This enables the method of an embodiment of the invention to be simplified, and the total time taken to treat the part by the method is shortened. 

What is claimed is:
 1. A method of treating a part made of aluminum alloy containing copper at a content lying in the range 0.1% to 10% by weight, wherein the method comprises: a. providing said part; b. performing electrochemical pretreatment of said part in a first electrolyte bath containing sulfuric acid and a first oxidizer compound, a first potential difference ΔV1 being established between a first cathode and a first anode dipped in said first bath, said part being said first anode, the concentration of said first oxidizer compound being such that the corrosion potential of said aluminum alloy is greater than +100 mV relative to a hydrogen normal electrode; c. after step b, anodizing said part in a second electrolyte bath containing sulfuric acid and a second oxidizer compound, a second potential difference ΔV2 being established between a second cathode and a second anode dipped in said second bath, said part being said second anode.
 2. A method according to claim 1, wherein, in step b, said first potential difference ΔV1 lies in the range 3 V to 12 V.
 3. A method according to claim 1, wherein said first oxidizer compound is hydrogen peroxide H₂O₂.
 4. A method according to claim 1, wherein anodizing step c is performed in the same bath as the first bath of the pretreatment step b, without extracting said part from said first bath between step b and step c.
 5. A method according to claim 1, wherein between step b and step c, a step is performed of rinsing said part.
 6. A method according to claim 5, wherein said rinsing is constituted by: b1. rinsing said part in a static rinsing bath; and b2. then rinsing said part in a running rinsing bath with softened water. 